This invention relates to a method of reducing the loss of carbon from anodes when producing aluminum by electrolytic smelting, and an inert anode for performing such method.
In producing aluminum by electrolytic smelting/reduction, carbon bodies, so called anode carbons, are used as anodes. During the reduction process, gases are liberated which oxidize the anode which is thus consumed and which must therefore be renewed.
Since the early beginnings of the aluminum industry, anodes have been in the form of carbon blocks fixed to a steel rod, so called anode rods. The mechanical and electrical connection is achieved by placing the anode rods, or extensions of these, studs, into holes in the upper side of the carbon block, fixing them there, either by casting them into position with molten iron, or tamping them with a carbon-based tamping paste.
In all cases, the carbon body has to be replaced when it has been consumed to such extent that there is a danger of the anode rod, or stud, being attacked by the electrolytic bath. When replaced, the unused part of the anode carbon will be 10-20 percent by weight of the anode when new.
It is immediately obvious that it is of great economic importance to reduce this carbon loss. Reducing the carbon loss will mean better utilization of the production plant for carbon bodies, and there will be less carbon waste to deal with.
Several methods are known for reducing anodic carbon loss.
Soderberg anodes are based on a special method whereby a continuous electrode is produced, i.e. with no carbon loss, in that the anode is produced by a continuous process in the electrolytic cell itself. For various reasons, the Soderberg anode is now being abandoned for the production of aluminum by electrolytic reduction.
In Germany, VAW developed a pre-baked anode which was to be capable of use without any waste. The anode was divided up into a number of carbons, the length of each carbon being the same as the breadth of the anode. Each anode carbon was fixed to an anode bus running along each long side of the anode, by means of copper bars screwed into the bus, two for each end of each anode carbon.
Renewing the anode was achieved by placing a new anode carbon on top of the old. The new carbon had a gluing sole whereby the remains (the butt) of the old anode carbon would remain suspended and be consumed when the new anode anode carbon was put into service by disconnecting the copper bars from the old carbon and attaching them to the new.
In earlier German Pat., BRD Nos. 863,999 and 1,090,867, the adhesive paste which was used in the arrangement in question was described. A non-homogeneous layer of the thickness in question (at least 5 mm) gives rise to considerable problems with respect to the passage of the electric current, in addition to the selective oxidization of the binder phase when the joint is consumed in the wearing surface over a period of about 48 hours.
This type of pot has proved difficult in practice, particularly when producers change over to electrolytic cells operating at higher currents. This type of pot has therefore been abandoned.
In principle, the type of anode used today is the same as it was in the early days of the aluminum industry, but the individual anode carbons are larger and usually held by two or three contact studs in each anode carbon.